Dry type transformers with primary voltages over 600 volts have generally been constructed using one of three types of techniques, conventional dry, resin encapsulated, or solid cast. The conventional dry method uses some form of vacuum impregnation with a solvent type varnish on a completed assembly consisting of the core and the coils or individual primary and secondary coils. Some simpler methods required just dipping the core and the coils in varnish without the benefit of a vacuum. The resulting voids or bubbles in the varnish that are inherently a result of this type of process due to moisture and air, does not lend itself to applications above 600 volts. The resin encapsulated method encapsulates a winding with a resin with or without a vacuum but does not use a mold to contain the resin during the curing process. This method does not insure complete impregnation of the windings with the resin and therefore the turn to turn insulation and layer insulation must provide the isolation for the voltage rating without consideration of the dielectric rating of the resin. The solid cast method utilizes a mold around the coil which is the principal difference between it and the resin encapsulated method. The windings are placed in the mold and impregnated and/or encapsulated with a resin under a vacuum, which is then allowed to cure before the mold is removed. Since all of the resin or other process material is retained during the curing process, there is a greater likelihood that the windings will be free of voids, unlike the resin encapsulated method whereby air can reenter the windings as the resin drains away before and during curing. Cooling channels can be formed as part of the mold. One type of such a transformer is manufactured by Square D Company under the trademark of Power-Cast transformers. Another example of a cast resin transformer is disclosed in U.S. Pat. No. 4,488,134.
Since the resin coating on solid cast coils results in a solid bond between adjacent conductors than is possible with resin encapsulated coils, solid cast coils exhibit better short circuit strength of the windings. Because the conductors in the coils are braced throughout by virtue of the solid encapsulant there is less likelihood of movement of the coils during short circuit conditions and short circuit forces are generally contained internally. External bracing, foil-wound coils, or selective geometry in the shape of the coils must be used in the resin encapsulated method to prevent movement of the coils caused by the forces of short circuit faults. An added benefit is that by having greater mass, there is a longer thermal time constant with the solid cast type coils and there is better protection against short term overloads. The resin encapsulated method does however have several distinct advantages over solid cast coils. They are simpler to manufacture and require less resin and other materials, resulting in less weight and lower costs. Additionally, the cast resin process requires an epoxy resin which also requires fillers such as glass fibers to provide mechanical strength. The epoxy resins generally are limited to a 185 deg. C. temperature, whereas resin encapsulated coils can utilize polyester resins which can achieve 220 deg. C. ratings. Given these advantages, it would be desirable to produce transformers with the resin encapsulated method if there were a method to increase the strength of the coil windings to prevent movement during short circuits. It would also be advantageous to provide better insulation at the top and bottom portions of the coils to prevent moisture and other environment contaminants from deteriorating the windings.
The air gap between the high and low voltage coils is dependent on having the same geometry between the outer surface of the inner coil and the inner surface of the outer coil. A large factor on the shape of the coil is the method of attaching the external leads to the winding. For non-molded coils, there is generally a distinct bulge at the point where this occurs. As a result, the air gap between coils will be uneven. Inductive reactance of a transformer is determined by this air gap, along with the number of turns in the coil and the physical dimensions of the coil. Controlling these factors will result in limiting short circuit currents and thus controlling withstand ratings.